Of the 1.7 million wheelchair users in the United States (US), 90 percent, or 1.5 million persons use manual wheelchairs (MWCs). People with traumatic and non-traumatic spinal cord injuries (SCI) make up approximately 20% of the MWC users, and 12,000 new traumatic SCIs occur every year. While MWCs are immediately available and enable independence, 63% of MWC users will have one or multiple rotator cuff tears after decades of WC use as compared to 15% of age-matched able-bodied adults. A crucial gap in knowledge exists in understanding how the shoulder of MWC users functions from an almost permanent seated position as an agent for mobility, weight bearing, and hand grasping; and how this altered function translates to longitudinal shoulder health decline. The objective of this application is to define the longitudinal components of the early phase (before chronic symptom onset) of shoulder health decline specific to new MWC users by characterizing the exposure to altered shoulder function and the associated MRI signs of early onset of shoulder pathology. We propose to: (Aim 1) quantify shoulder joint motion and loading in the real world over 3 years in 60 new MWC users and a matched able-bodied cohort; (Aim 2) define early, preclinical changes on shoulder MRI specific to the MWC users, over 3 years, in comparison to the matched cohort; and (Aim 3) identify specific exposure measures as risk factors for early changes on MRI in the MWC users. Three central and novel aspects of this proposal will pave the way for targeting primary prevention: (1) characterizing the altered shoulder function in new MWC users in the real world with hardware and instrumentation suitable for multiple, day long collections, (2) defining the early, preclinical pattern of disease in users compared to a matched able-bodied cohort, and (3)investigating the combined effect of shoulder motion and loading and its relationship to the incidence of shoulder pathology. Successful completion of this project will define how the shoulder responds to MWC use (Aim 1), identify the MWC-specific pattern of shoulder disease on MRI (Aim 2), and determine how altered shoulder function has contributed to shoulder health decline (Aim 3). This work provides the foundation for understanding the relative impact of shoulder elevation and loading in shoulder health decline. Additionally, this work provides the first building block in defining the complete natural history of shoulder disease in MWC users. We expect the overall impact to be a powerful influence on environmental and assistive technology redesign, post-SCI rehabilitation practices, insurance reimbursement for shoulder health- preserving equipment, and understanding shoulder pathology in the general population.